DN426 - 6-Channel SAR ADCs for Industrial Monitoring and Portable

6-Channel SAR ADCs for Industrial Monitoring and Portable
Instruments
Design Note 426
Guy Hoover and Steve Logan
The 14-bit LTC2351-14 is a 1.5Msps, low power SAR
ADC with six simultaneously sampled differential input
channels. It operates from a single 3V supply and features
six independent sample-and-hold amplifiers and a single
ADC. The single ADC with multiple S/HAs enables excellent
range match (1mV) between channels and channel-tochannel skew (200ps).
The versatile LTC2351-14 is ideally suited for industrial
monitoring applications such as 3-phase power line
monitoring to ensure line voltage compliance, portable
power line instrumentation, power factor correction, motor control, and data acquisition. These applications may
be battery powered, and it is here that the LTC2351-14’s
low power and small size are desirable. Power consumption is a mere 16.5mW, which extends battery life. The
3-wire serial interface means fewer pins than parallel
output devices, allowing the LTC2351-14 to fit in a 32-pin,
5mm × 5mm QFN package.
Power Line Monitoring Application
Figure 1 shows a typical power line monitoring application.
Current is sensed by a CR Magnetics CR8348-2500-N
current transformer. An LT1790-1.25 biases the output
of the transformer to the middle of the LTC2351-14 input
range, giving the inputs maximum swing. A 6:1 transformer and 41:1 attenuator scale the line voltage, and the
transformer output is similarly biased.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
LT1790-1.25
IN
OUT
3V
0.1MF
3V
1MF
GND
10MF
100Ω
CH0
49.9Ω
4 TURNS
+
47pF
47pF
49.9Ω
CR MAGNETICS
CR8348-2500-N
0.1MF
VCC VDD OVDD
LTC2351-14
VREF
S&H
100Ω
47pF
2k
6:1
MUX
49.9Ω
+
47pF
47pF
49.9Ω
49.9Ω
14-BIT ADC
CH1
49.9Ω
120VAC
LINE
2k
10MF
–
S&H
TO
PROCESSOR
–
47pF
CHANNELS 2-5 TO MEASURE TWO OTHER VI PAIRS
Figure 1. Typical Power Line Monitoring Application
10/07/426
CONV
SCK
SDO
DN426 F01
Figure 2 shows the AC line voltage in Linear Technology’s
Mixed Signal lab. The flattened peaks are typical of
the voltage in an office building where many of the
loads are nonlinear, such as computer power supplies.
Figure 3 shows the current through a 50W incandescent
bulb. Figure 4 shows the current through a 15W compact
fluorescent bulb, and Figure 5 is the current through a 4W
LED-based bulb. The 5MHz full linear bandwidth of the
LTC2351-14 allows analysis of high frequency components
of the line voltage and current, limited in this case by the
bandwidth of the sense transformers.
Conclusion
With PCB real estate getting tighter and designers always
searching for lower power ICs, fast data acquisition can
be a challenge. The LTC2351-14 and other low power
SAR converters make it possible to optimize solution
size, power and cost.
200
1.0
150
0.8
0.6
0.4
50
CURRENT (A)
VOLTAGE (V)
100
0
–50
0.2
0
–0.2
–100
–0.4
–150
–0.6
–200
–0.8
–1.0
–250
0
10
20
30 40 50
TIME (ms)
60
70
0
80
10
20
30 40 50
TIME (ms)
60
Figure 3. 50W Incandescent Bulb Current
1.0
1.0
0.8
0.8
0.6
0.6
0.4
0.4
CURRENT (A)
CURRENT (A)
Figure 2. Line Voltage
0.2
0
–0.2
0.2
0
–0.2
–0.4
–0.4
–0.6
–0.6
–0.8
–0.8
–1.0
10
20
30 40 50
TIME (ms)
60
80
dn426 F03
dn426 F02
0
70
70
80
dn426 F04
Figure 4. 15W Compact Fluorescent Bulb Current
–1.0
0
10
20
30 40 50
TIME (ms)
60
70
80
dn426 F05
Figure 5. 4W LED Bulb Current
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 2602
www.linear.com
Linear Technology Corporation
dn426f LT/TP 1007 305K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2007
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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